The present invention relates to a steam-raising system comprising a boiler having (a) a passageway for water and/or steam, (b) an inlet to the passageway through which water is introduced continuously for given periods when the boiler is in use, (c) a burner to heat the passageway from the outside thereof, and (d) an outlet from the passageway from which steam emerges continuously as water is introduced through the inlet, the system further comprising a pump connected by a flow-path to the said inlet to pump water thereto along the said flow-path.
EP-A-727,609 describes such a system with flow-control means comprising a plurality of lines which constitute a part of the flow-path, which are connected between the pump and boiler in parallel with one another, and which are independently openable to enable the amount of water delivered to the boiler to be varied, each line having a shut-off valve and a flow regulator which maintains a constant flow through it substantially independently of the pressures upstream of its inlet and downstream of its outlet.
A disadvantage of this previously proposed system is the relatively crude control obtained by switching in or out one or more of the lines, and the relatively high expense involved in having a shut-off valve and a further regulator for each line.
The present invention seeks to obviate this disadvantage.
Accordingly, the present invention is directed to a steam raising system as set out in the opening paragraph of the present specification, in which the pump is a variable-flow-rate pump constructed to provide a flow-rate through it which is substantially independent of its downstream pressure.
Advantageously, heated water is fed to the pump, preferably via a further pump upstream of the first-mentioned pump to create an upstream pressure sufficient to avoid cavitation therein.
Conveniently, the water fed to the first-mentioned pump is heated by means of some of the steam which emerges from the boiler.
The first-mentioned pump may be an electrically operated pump, preferably operated by an alternating electrical current, advantageously constructed so that the flow-rate is dependent upon the frequency of the alternating current. In that case control means may be provided to vary the frequency in dependence upon the required flow-rate of water to the boiler.
The first-mentioned pump may comprise a plurality of piston and cylinder arrangements, each connected to draw in water from a water source during a first stroke and then to pump it through an outlet of the pump during a second stroke, the arrangements being in suitably different phases relative to one another to obtain a continuous supply of water from the outlet.
The pistons may be moved by means of a rotary surface which slants relative to an axis of the pump to which at least components of the longitudinal extent of the cylinders of the piston and cylinder arrangements are substantially parallel.
Temperature monitoring means may be provided downstream of the boiler to measure the temperature of the steam output of the boiler.
The temperature monitoring means may be connected to a control unit of the system which adjusts the flow-rate of the first-mentioned pump in dependence upon the measured temperature of the steam output of the boiler.
The control unit may thereby maintain a given flow-rate or a given head of steam output.
The present invention extends to a method of raising steam by a system in accordance with the present invention.